THE WORKS OF FRANCIS MAITLAND BALFOUR. VOL. II. Memorial Edition. Cambridge: PRINTED BY C. J. CLAY, M.A. AND SON, AT THE UNIVERSITY PRESS. Memorial Edition. THE WORKS OF FRANCIS MAITLAND BALFOUR, M.A., LL.D., F.R.S., FELLOW OF TRINITY COLLEGE, AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF CAMBRIDGE. EDITED BY M. FOSTER, F.R.S., PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE; AND ADAM SEDGWICK, M.A., FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE. VOL. II. A TREATISE ON COMPARATIVE EMBRYOLOGY. Vol. I. Invertebrata. London: MACMILLAN AND CO. 1885 [_The Right of Translation is reserved._] PREFACE. My aim in writing this work has been to give such an account of the development of animal forms as may prove useful both to students and to those engaged in embryological research. The present volume, save in the introductory chapters, is limited to a description of the development of the Invertebrata: the second and concluding volume will deal with the Vertebrata, and with the special histories of the several organs. Since the work is, I believe, with the exception of a small but useful volume by Packard, the first attempt to deal in a complete manner with the whole science of Embryology in its recent aspects, and since a large portion of the matter contained in it is not to be found in the ordinary text books, it appeared desirable to give unusually ample references to original sources. I have accordingly placed at the end of each chapter, or in some cases of each section of a chapter, a list of the more important papers referring to the subject dealt with. The papers in each list are numbered continuously, and are referred to in the text by their numbers. These lists are reprinted as an appendix at the end of each volume. It will of course be understood that they do not profess to form a complete bibliography of the subject.